The flow rate of a fluid flowing in a flow system at slow to moderate rates can be measured by placing a magnetically responsive body in a flow conduit having a stop against which the body rests under the influence of the fluid flow (and gravity in a vertically downward or sloping mode) and determining the force required to be exerted on the body by an electromagnet located upstream from the stop to move the body away from the stop. With the body resting on the stop, the current supplied to the electromagnet is increased from zero (or a level below that which causes the body to leave the seat). When the current reaches a certain level, the magnetic force applied to the body becomes large enough to move the body upstream against the flow. Inasmuch as the magnetic force exerted on the body rapidly increases as soon as the body starts moving toward the electromagnet, the body rapidly moves against the flow into a position between the magnetic poles. By measuring accurately the current supplied to the electromagnet when the body moves away from the stop, one has a value that can be converted into a flow rate value based on calibration data. Movement of the body away from the stop has previously been detected by optical sensors. U.S. Pat. No. 4,003,255 (Spencer, Jan. 1977, "the '255 patent") describes and shows a flow measuring system of the type just described. The '255 patent is incorporated by reference into the present disclosure.
Other fluid flow measuring systems based on magnetic attraction of magnetically responsive bodies and optical sensing of movements of the bodies have been proposed, such as those of U.S. Pat. Nos. 4,041,723 (Head et al., October 1977); 3,662,598 (Spencer, May 1972); 3,605,741 (Spencer, September 1971); and 4,167,115 (Stoever, September 1979).
Optical sensing requires that the flow conduit and the fluid be transparent and is also subject to malfunction due to false indications of ball movement caused by ambient light incident on the light detector. Ambient light conditions can, for example, cause inaccuracies in the measurements due to variations in the position of the body along the path it takes between the stop and the magnet when a signal indicative of the movement of the body is generated.